Birefringent lens for laser beam delivery

ABSTRACT

An ophthalmic laser system includes a laser engine to generate a beam of femtosecond laser pulses, a laser scanner to scan each laser pulse of the beam in three dimensions according to a scan pattern, and a compound lens comprising a glass lens and a birefringent lens, the compound lens arranged to receive the scanned beam and configured to split each laser pulse of the scanned beam into an ordinary pulse and an extraordinary pulse, producing an ordinary beam comprising ordinary pulses and an extraordinary beam comprising extraordinary pulses. A particular ordinary pulse and a particular extraordinary pulse split from a particular laser pulse are spatially separated in depth along an optical axis of the compound lens, by a distance greater than or equal to 5 μm, and temporally separated by a delay greater than or equal to a pulse duration of the femtosecond laser pulses. An objective is configured to focus the ordinary beam and the extraordinary beam within an ophthalmic target.

FIELD

This present disclosure relates generally to femtosecond laser systemsand, more particularly, to femtosecond laser systems for ophthalmicsurgical procedures.

BACKGROUND

Surgery with femtosecond ophthalmic lasers entails generating a pulsedlaser beam and delivering the laser pulses by a scanning delivery systemthrough a focusing optics to a sequence of focus spots along a scanpattern in a target region of an ophthalmic tissue. Each laser pulse cancreate a plasma or cavitation bubble in the target tissue at the focusspot of the laser beam when the beam intensity or energy density exceedsa plasma or photodisruption threshold. During surgery, the focus spot ofthe laser beam is scanned along a three-dimensional scan pattern,creating a sequence of these bubbles to form macroscopic surgical cutsor photodisrupted regions.

Laser-assisted cataract surgery (LACS) requires the applicationtypically a few million laser pulses. The LenSx® femtosecond lasersystem manufactured by Alcon® is capable of completing a typical LACSprocedure which includes capsulotomy and lens fragmentation in about oneminute. During such procedures, the patient lays prone beneath the lasersystem while the eye is immobilized by a patient interface. Theaccuracy, precision, and patient experience of LACS can be improved byshortening the procedure time. Accordingly, there is a need for simple,inexpensive, and reliable technique to reduce procedure times oflaser-assisted ophthalmic surgeries.

SUMMARY

In general, the present disclosure relates to a multifocal laser beamdelivery system for femtosecond laser systems. Certain embodimentsprovide a laser system that includes a laser engine configured togenerate a beam of femtosecond laser pulses, a laser scanner configuredto scan each laser pulse of the beam in three dimensions according to ascan pattern, and a compound lens (e.g., a doublet lens, triplet lens,etc.) comprising a glass lens and at least one birefringent lens. Thecompound lens is arranged to receive the scanned beam and configured tosplit each laser pulse of the scanned beam into an ordinary pulse and anextraordinary pulse, thereby producing an ordinary beam comprisingordinary pulses and an extraordinary beam comprising extraordinarypulses. A particular ordinary pulse and a particular extraordinary pulsesplit from a particular laser pulse are spatially separated in depthalong an optical axis of the compound lens, by a distance greater thanor equal to 5 μm, and temporally separated by a temporal delay greaterthan or equal to a pulse duration of the femtosecond laser pulses. Thesystem includes an objective configured to focus the ordinary beam andthe extraordinary beam within an ophthalmic target.

In variants of the disclosed system the scan pattern may be a spiral orraster scan pattern, and the birefringent lens may comprise abirefringent crystal. Embodiments may include a rotation lens mountcoupled to the birefringent lens, such that the birefringent lens isrotatable to adjust a relative energy content of the ordinary beam andthe extraordinary beam. The birefringent lens may be configured suchthat rotating the birefringent lens to a first position causes therelative energy content between the ordinary beam and the extraordinarybeam to be equally distributed, and rotating the birefringent lens to asecond position causes at least 95%, 98%, or 99% of the relative energyto be distributed to the ordinary beam and no more than 5%, 2%, or 1% ofthe relative energy to be distributed to the extraordinary beam.

Embodiments may include a rotation mount coupled to a halfwave platepositioned between the scanner and the birefringent lens, such that thehalfwave plate is rotatable to rotate the polarization of the scannedbeam and thereby adjust the relative energy content of the ordinary beamand the extraordinary beam. The halfwave plate may be configured suchthat rotating the halfwave plate to a first position causes the relativeenergy content between the ordinary beam and the extraordinary beam tobe equally distributed; and rotating the halfwave plate to a secondposition causes at least 95%, 98%, or 99% of the relative energy to bedistributed to the ordinary beam and no more than %, 2%, or 1% of therelative energy to be distributed to the extraordinary beam.

In some variants of the disclosed system the spatial separation in depthof the ordinary and extraordinary pulses along the optical axis is inthe range of 5-100 μm, and the delay temporally separating the ordinaryand extraordinary pulses is in the range of 100-1000 femtoseconds.

In certain embodiments, the system comprises a laser controllerconfigured to generate a scan pattern based on a rotation position ofthe birefringent lens or halfwave plate, and control the scanner to scanthe beam according to the generated scan pattern. The scan pattern maybe a three-dimensional spiral scan pattern, or a raster scan pattern.

In certain embodiments, the laser engine is configured to generate thebeam of femtosecond laser pulses with a pulse energy sufficient to causephotodisruption in an ophthalmic target at focus spots of the ordinarybeam and the extraordinary beam. The compound lens may be configured sothat the spatial and temporal separation between the particular ordinarypulse and particular extraordinary pulse is sufficient to cause aphotodisruption at a first depth in the ophthalmic target before causingphotodisruption at a second depth in the ophthalmic target, wherein thefirst depth is greater than the second depth relative to an externalsurface of the ophthalmic target.

In variants of the disclosed system, the birefringent lens may be madeof calcite (CaCO₃), ytrium orthovanadate (YVO₄), lithium niobate(LiNbO₃), barium borate (BaB₂O₄), or other birefringent crystalmaterial.

Embodiments of the disclosure provide one or more advantages andbenefits. For example, the z-doubling technique described herein can beused to effectively increase the frequency of a pulsed laser by a factorof 2λ, without increasing the actual scan rate. Accordingly, embodimentsprovide a simple and reliable optical approach for increasing scanningrate without modifying the laser engine or beam scanner (eliminatingpotential galvo speed issues, pockels cell issues, etc. attendant todoubling the repetition rate by conventional means). It can be appliedand tested in existing machines with minimal modification.

Additionally, embodiments are configured to focus two laser spots inclose enough proximity (e.g., ≈25 μm) to improve lens fragmentationefficiency by a factor of more than 2×. Even more, the z-doubling effectof the birefringenent lens can be “turned on” or “turned off” byrotating the plane of polarization of the beam incident on the lens, orrotating the birefringent lens itself. Hence, the relative energycontent of the ordinary and extraordinary foci can be easily controlledby mounting the birefringent lens or a halfwave plate on a rotating lensmount. This enables simple reconfiguration of a machine for differentprocedures (e.g., cataract and refractive procedures), and allows a userto control the direction of propagation of expelled gas during anophthalmic procedure where z-doubling is employed.

Further, because the shallow focus spot is delayed in time relative tothe deeper focus spot, shadowing effects caused by plasma generationwill not impede procedures which employ the z-doubling techniquesdescribed herein.

These and other benefits will be apparent to one skilled in the art inview of the drawings and specification

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numerals indicate like features and wherein:

FIG. 1 is a block diagram of a laser system, according to certainembodiments;

FIG. 2 is a schematic of components of delivery optics in a lasersystem, according to certain embodiments;

FIGS. 3A and 3B are diagram illustrating effects of shadowing caused bya focal spot;

FIG. 4 depicts a method, according to certain embodiments;

FIG. 5 is a schematic of components of delivery optics in a lasersystem, according to certain embodiments.

One skilled in the art will understand that the drawings, describedbelow, are for illustration purposes only. The drawings are not intendedto limit the scope of the claims or disclosure.

DETAILED DESCRIPTION

The following description is presented to enable one skilled in the artto make and use the inventions disclosed herein, and is generallyprovided in the context of a particular application and itsrequirements. Various modifications to the disclosed embodiments will beapparent to those skilled in the art, and it is understood that generalprinciples set forth herein may be applied to other embodiments andapplications without departing from the spirit and scope of thedisclosure. It is further understood that systems, devices, components,and methods described with respect to one embodiment may be combinedwith features, components, and/or steps described with respect to otherembodiments. For the sake of brevity, however, the numerous iterationsof these combinations will not be described separately. Thus, the scopeof the disclosure is not limited to the embodiments described, but is tobe accorded the widest scope consistent with the claims.

In general, the present disclosure relates to an improved multifocallaser beam delivery system for femtosecond laser systems. In certainembodiments, a compound lens (e.g., a doublet or triplet lens)comprising at least one birefringent lens is included in the deliveryoptics of a femtosecond laser. The compound lens can split the laserenergy generated by a laser engine into an ordinary and an extraordinarybeam which may be focused “below” each other (separated in the depthdirection, along the optical axis of the laser system) so that the laserspot of each beam forms a breakdown cavity in the target (such asophthalmic tissue to be treated). This may be referred to as“z-doubling.” In this way, the effective laser repetition rate can bedoubled without increasing the repetition rate of the laser engine,thereby avoiding technical challenges which accompany doubling therepetition rate of a laser source. Additionally, the compound lens mayimpart a delay so that the ordinary and extraordinary beams aretemporally separated, forming a breakdown cavity at a first depth beforeforming a breakdown cavity at a second, shallower depth in the target.In this manner, any distortion and shadowing resulting from opticalbreakdown at the second depth will not interfere with optical breakdownsat the first depth. Without such a delay, the plasma generated at thesecond (shallower) depth may cast a shadow which interferes with thefocal spot at the first (deeper) depth, impeding formation of abreakdown cavity at the first depth. Accordingly, embodiments provide asimple and reliable solution for increasing the effective repetitionrate of a femtosecond laser system.

FIG. 1 is a block diagram of an example ophthalmic surgical laser system100. The system includes a pulsed laser source 110, scanners 120,delivery optics 130, patient interface 140, imaging device 150, andlaser controller 160. One skilled in the art will appreciate that system100 may include additional components which are not shown for the sakeof simplicity.

Pulsed laser source 110 (also referred to as laser engine 110) maycomprise a laser engine capable of generating a beam 101 of femtosecondlaser pulses. In certain variants, laser engine 110 comprises a chirpedpulse amplification (CPA) laser which includes an oscillator to generatefemtosecond seed pulses, a stretcher to stretch the seed pulses by afactor of 10-1000 to the picosecond range, an amplifier to amplify thepulses, and a compressor to compress the length of the amplified pulsesback to the femtosecond range. In certain examples, laser engine 110 maybe a cavity-dumped regenerative amplifier laser which includes anoscillator, stretcher-compressor, and optical amplifier. Laser engine110 may, in certain variants, comprise a bulk laser, fiber laser, orhybrid laser design. In certain variants, the laser pulses generated bylaser engine 110 may have a pulse duration in the range of 100-10,000femtoseconds, 600-5000 femtoseconds, or 600-1000 femtoseconds, aper-pulse energy in the range of 0.1-1000, and a repetition frequency inthe 1 kHz-1 MHz range. In various examples, generated laser pulses mayhave laser pulse parameters which fall within a combination of theserange limits, such as a range of pulse duration of 10 fs-10 ps Specificlaser parameters for a particular procedure may be selected within suchranges based on patient-specific and/or procedure-specific data. Whensplitting pulses (e.g., z-doubling) for a procedure such as a cataractprocedure, each generated laser pulse has a total energy that is atleast two times (2×) the photodisruption threshold of the target tissue,so that each ordinary and extraordinary pulse produced by a compoundlens 200 has sufficient energy to independently induce photodisruptionat ordinary and extraordinary focal spots.

Scanners 120 includes any suitable combination of x-y scanners andz-scanners (which may include steering mirrors, galvanometers, lenses,servomotors, etc.) which scan the beam 101 to direct focal spots to aseries of points in a three-dimensional surgical scan pattern in thetarget tissue. Each laser pulse can create a plasma or cavitation bubblein an ophthalmic target 103 at a focal spot 102 of beam 101 when theintensity or energy density exceeds a plasma or photodisruptionthreshold. Scanners 120 thus control where photodisruption occurs in thetarget, and scan focus spot 102 in three dimensions within target 103 toexecute a surgical plan. An x-y scanner scans focus spot 102 of beam 101in an x-y plane perpendicular to an optical axis of the laser system100, while a z-scanner scans focus spot 102 of beam 101 in thez-direction to particular depths in target 103 along the optical axis ofthe laser system 100. Various embodiments may include one or morez-scanners positioned between pulsed laser 100 and an x-y scanner,between the x-y scanner and delivery optics 130, or both. In someembodiments, a z-scanner may additionally or alternatively be locatedwithin delivery optics 130. Scanners 120 may be configured to scan beam101 in any suitable surgical pattern, including spiral patterns, rasterpatterns, elliptical patterns, circular patterns, spider patterns, etc.The scan patterns may be automatically and/or manually generated by aprocessor of laser controller 160 based on patient andprocedure-specific data and forwarded to scanner 120 via control signals106.

Delivery optics 130 may include a focusing objective lens, beamexpander, birefringent lens (discussed further below), and other lensesto direct, collimate, condition, and/or focus the scanned beam 101through the patient interface 140 to focus spot 102 in target 103.Components of delivery optics 130 may be slidably or movably mounted ona base so that the weight of the objective lens and other opticalcomponents is balanced to limit force applied on patient interface 140and target 103. Patient interface 140 may include, for example, a one ortwo-piece transparent applanation lens attached to a mount on deliveryoptics 130. The mount can provide a stable connection between thepatient interface and delivery optics 130. Patient interface 140 mayattach to and immobilize a target such as an eye during a laserprocedure.

System 100 may additionally include one or more imaging devices 150. Incertain embodiments, system 100 includes a surgical microscope, videomicroscope, digital microscope, ophthalmoscope, and/or camera to receiveimaging light 104 and generate live images of the surface of target 103during a procedure. System 100 may also include enhanced imaging devicesto assist in guiding the laser surgery. In one example, imaging device150 includes an optical coherence tomography (OCT) imaging system togenerate depth-resolved images of the inner structure of target 103,such as the location, position, and curvature of the crystalline lens,the anterior and posterior capsules, and the cornea. Imaging device 150may comprise any suitable SS-OCT, SD-OCT, or TD-OCT system forgenerating static or live A-scan, B-scan, or 3D OCT images. Imagingdevices 150 may comprise x-y and z-scanners to scanning an imaging beaminto target 103 (not shown).

Image data 105 generated by imaging device 150 may be provided to alaser controller 160. Laser controller 160 comprises memory (“M”)storing instructions executable by a processor (“P”) to control pulsedlaser source 110, scanners 120, delivery optics 130, and/or imagingdevices 150. Typically, the processor of laser controller 160 comprisesone or more CPUs (such as those manufactured by Intel, AMD, and others),microprocessors, field-programmable gate arrays (FPGAs),application-specific integrated circuits (ASICs), digital-signalprocessors (DSPs), or system-on-chip (SoC) processors communicativelycoupled to memory. The memory may comprise a non-transitorycomputer-readable medium, and may include volatile or non-volatilememory including, magnetic media, optical media, random access memory(RAM), read-only memory (ROM), removable media, or analogous components.The memory may store software instructions executable by the processorto generate control signals 106 that control the operation of pulsedlaser source 110, scanners 120, delivery optics 130, and imaging device150.

For example, laser controller 160 may generate signals 106 to controlparameters of beam 101 generated by pulsed laser source 110, such as arepetition rate, pulse length, and pulse energy. Laser controller 160may also generate signals 106 to actuate individual components ofscanners 120 in order to direct focus spot 102 according to a surgicalscan pattern. Such scan patterns may be any suitable 2D or 3D shape orpattern, including spiral, raster, circular, elliptical, cylindrical, orspider patterns. For example, the scan pattern may be athree-dimensional spiral scan pattern in which the laser beam is focusedto particular spots in an x-y plane in a spiral sequence at variousz-depths within the cylinder. In certain examples, laser controller 160generates scan patterns to create a capsulotomy cut in the anteriorcapsule of a target 103 and fragment a crystalline lens of target 103prior to extraction. Laser controller 160 may generate control signalsbased on image data 105 (e.g., OCT images) obtained intraoperativelyduring a surgical procedure. In this manner, system 100 can performimage-guided ophthalmic surgery based on real-time feedback.

It is noted that, to the extent used in the claims below, the terms“processor,” and “memory” refer to structures specially configured for afemtosecond laser system, as known to those skilled in the art, andthose terms should not be construed or interpreted asmeans-plus-function elements.

FIG. 2 illustrates aspects of delivery optics 130 according to certainembodiments. Delivery optics 130 may include compound lens 200comprising lens 210 and at least one birefringent lens 220, beamexpander 230, and objective 240. In the illustrated example, compoundlens 200 comprises a doublet lens. In other embodiments, compound lens200 may comprise a triplet lens, or comprise additional lenses. Pulsedbeam 101 generated by laser source 110 and scanned by scanners 120propagates along optical axis 250 to compound lens 200, which splits thebeam into an ordinary beam 101 o and extraordinary beam 101 e havingdifferent wavefront curvatures and temporal delays.

It is noted that, in various embodiments, the arrangement of opticalelements in delivery optics 130 may vary. For example, compound lens 200could be placed between beam expander 230 and objective 240 in certainvariants, or between multiple lenses of beam expander 230.

Ordinary and extraordinary beams 101 o and 101 e arrive at beam expander230, which comprises one or more lenses which increase the diameter ofbeams 101 o and 101 e. Beam expander 230 may be a movable or fixed blockof lenses and in some embodiments may include a z-scanner. In certainembodiments, positioning lens 200 at different locations within beamexpander 230 can determine the relative separation of ordinary andextraordinary focus spots 250 o and 250 e without affecting the temporaldelay between them, as discussed with respect to FIG. 5 below.

Expanded beams 101 o and 101 e arrive at objective 240. Objective 240may comprise a focusing objective with one or more lenses having a totalfocal length F. Objective 240 focuses beams 101 o and 101 e to focusspots 250 o and 250 e that are spatially separated in depth by adistance Δ along optical axis 250 and temporally separated by a delaytime. The objective 240 can focus the expanded laser beam into a highnumerical aperture (high NA) focused beam and deliver it to the targetregion through patient interface (PI) 140. Generating a high NA beam canensure that the ordinary and extraordinary beams cause photodisruptiononly at the intended z-depth or longitudinal (x-y) coordinate, thusavoiding collateral damage posterior or anterior relative to the targetregion. In some embodiments, NA can be in the 0.10-0.45 range. In someothers, it can be in the 0.25-0.35 range.

As noted above, compound lens 200 comprises complimentary lens 210 andat least one birefringent lens 220. In certain examples, lens 210comprises a glass, such as Schott BK7. In other examples, lens 210 maycomprise another material with suitable optical and mechanicalcharacteristics. Birefringent lens 220 may comprise birefringent crystalhaving an ordinary refractive index n_(o) and an extraordinaryrefractive index n_(e). Variants of birefringent lens 220 may be madefrom calcite (CaCO₃), ytrium orthovanadate (YVO₄), lithium niobate(LiNbO₃), and barium borate (BaB₂O₄), or other birefringent materials.

As noted above, in certain embodiments compound lens 200 may bedifferently positioned within the delivery optics. In one example, beamexpander 230 may comprise at least two lenses 230 a and 230 b, as shownin FIG. 5, and compound lens 200 may be adjustably positioned betweenlenses 230 a and 230 b. In this manner, the z-separation (distance)between ordinary and extraordinary focus spots 250 o and 250 e can becontinuously varied. In different arrangements, the magnitude and sign(positive or negative) of a a z-separation (Δ shown in FIG. 2) can becontrolled in this manner. For example, when compound lens 200 is moved(e.g., by an actuator) to focus point 500 of beam expander 230, az-separation (Δ shown in FIG. 2) between the ordinary and extraordinaryfocus spots 250 o and 250 e will be zero. If compound lens 200 is moved(e.g., by the actuator) from point 500 toward the source of the laserbeam (to the left of focus spot 500 shown in FIG. 5—as illustrated), theordinary spot 250 o will be located optically upstream (in FIG. 5, tothe left) of extraordinary spot 250 e. Moving lens 200 to the other sideof focus spot 500 reverses this arrangement. Thus, if compound lens 200is moved (e.g., by the actuator) from point 500 away from the source ofthe laser beam (in FIG. 5, to the right of focus spot 500), theextraordinary spot 250 e will be located optically upstream (in FIG. 5,to the left) of ordinary spot 250 o. The temporal delay between spots250 o and 250 e is not affected by the position of lens 200 within beamexpander 230.

In the example shown in FIG. 2, lens 210 has a negative power andbirefringent lens 220 has a positive power. Although the disclosure isnot limited to the arrangement shown (either component lens 210 or 220may be negative, positive, or neither, according to particular designgoals), the design of lens 210 and birefringent lens 220 may beoptimized to split beam 101 into an ordinary beam 101 o andextraordinary beam 101 e with particular characteristics. For example,the focal lengths and refractive indices of lens 210 and birefringentlens 220 may be selected to impart particular wavefront curvature andtemporal delay to the ordinary and/or extraordinary beams. In certainvariants, lens 210 may comprise a material with a relatively lowrefractive index and low dispersion, and may be fabricated with apositive power to complement the refractive indices n_(o), n_(e) and anegative power of birefringent lens 220, in order to split beam 101 intoordinary and extraordinary beams 101 o and 101 e which are spatiallyseparated in depth along optical axis 250 and temporally separated by adelay.

The relative spatial separation in depth Δ of ordinary beam spot 250 oand extraordinary beam focus spot 250 e shown in FIG. 2 may be may beapproximated as follows:

Δ=[(n _(o) −n _(e))/(n _(e)−1)]*F ²/(M ² /f ₀)

Where:

n_(o) is an ordinary refractive index of birefringent lens 220;

n_(e) is an extraordinary refractive index of birefringent lens 220;

F is the focal length of objective 240;

M is the magnification of beam expander 230; and

f₀ is the focal length of lens 210.

Hence, in various embodiments, optical characteristics of the componentsshown in FIG. 2 may be optimized to separate focus spots 250 e and 250 oby a distance of, for example, 20 μm, 25 μm, or 30 μm. Other variantsmay separate focus spots 250 e and 250 o by distances in the range of10-40 μm, 15-35 μm, 20-30 μm, or other ranges.

In addition to separation in depth along the optical axis 250, it can bebeneficial to impart a temporal separation between the focus spots toeliminate problems which may arise from “shadowing.” FIG. 3A illustrateshow “shadowing” resulting from focusing an ordinary spot 250 o at a“shallow” depth can interfere with focusing an extraordinary spot 250 eat a greater depth along optical axis 250.

Generally speaking, photodisruption occurs when a laser pulse is focusedin ophthalmic tissue with sufficient energy to form plasma in the areaof the focal spot. Rapid expansion of the plasma is followed by emissionof a shock wave and formation of a cavitation bubble. As a result,tissue within the focal volume is destroyed. Multiple focal spotsapplied in a scan pattern can be used to create precise incisions inophthalmic tissue. However, the plasma formed at the focal spot is nottransparent. As a result, if ordinary beam 101 o and extraordinary beam101 e are simultaneously focused to spots 250 o and 250 e, as shown inFIG. 3A, shadowing 300 results when plasma generated by spot 250 oblocks part of extraordinary beam 101 e and prevents or impedesphotodisruption at focus spot 250 e.

Such shadowing effects can be avoided by configuring birefringent lens220 to introduce a temporal delay between the ordinary and extraordinarybeams, in addition to the spatial separation in depth discussed above.In certain examples, birefringent lens 220 is designed to delay pulsesof one or the other of ordinary or extraordinary beam by at least theduration of each pulse generated by the pulsed laser source 110. As aresult, plasma will develop at a spot of lesser depth (e.g., spot 250 o)only after the spot at the greater depth (e.g., spot 250 e) has beendelivered, as shown in FIG. 3B. In this manner, shadowing caused byplasma at focus spot 250 o does not affect photodisruption at focus spot250 e. Variants of birefringent lens 220 may be designed to induce arelative delay between ordinary and extraordinary beam pulses of atleast 300, 500, 800, 1000, 1200, 1500, or 2000 femtoseconds, or in therange of 100-10,000 femtoseconds.

Returning to FIG. 2, each pulse of pulsed beam 101 is split by compoundlens 200 to generate an ordinary beam 101 o having an ordinary wavefront201 o and ordinary beam focus spot 250 o, and an extraordinary beam 101e having an extraordinary wavefront 201 e and an extraordinary beamfocus spot 250 e (disregarding Fresnel reflection). The relative energycontent of the ordinary and extraordinary foci 250 o and 250 e may bedescribed as follows:

E _(e) =E*cos²(ϕ)

E _(o) =E*sin²(ϕ)

Where:

E_(e) is the energy of the extraordinary beam 101 e;

E_(o) is the energy of the ordinary beam 101 o;

ϕ is the angle of polarization of beam 101 with respect to the opticalaxis of the birefringent material;

and E is the total energy of beam 101 before it arrives at compound lens200.

Thus, the relative energy content of the ordinary and extraordinarybeams 101 o and 101 e can be controlled by rotating the angle ofpolarization of beam 101. For example, when the plane of polarization ofbeam 101 is rotated to ϕ=45°, the relative energy content of theordinary and extraordinary foci produced by splitting will be equal,such that the ordinary beam 101 o and extraordinary beam 101 e each haveapproximately 50% of the total energy of beam 101 when it arrives atcompound lens 200. Similarly, when the plane of polarization of beam 101is rotated to ϕ=0°, the relative energy content of the ordinary orextraordinary foci will approach a 100% to 0% split, effectivelyeliminating the multiplexing effect of birefringence (only one of theordinary or extraordinary focal spot will have sufficient pulse energyto cause photodisruption).

The angle of polarization ϕ may be adjusted in various ways. Forexample, birefringent lens 220 or compound lens 200 may be mounted intoa rotating lens mount such that rotating birefringent lens 220independently or compound lens 200 as a unit within the mount changesthe angle of polarization of beam 101. Additionally or alternatively,the angle of polarization of beam 101 may be rotated independently ofbirefringent lens 220 by including a half (λ/2) waveplate (not shown inFIG. 2) positioned in the path of beam 101 before it impinges onbirefringent lens 220. For example, a half wave crystal quartz waveplatemay be coupled to a rotating lens mount arranged between scanner 120 andcompound lens 220, such that rotating the waveplate changes the anglepolarization of beam 101 before it is received by compound lens 220.

Adjusting the angle of polarization ϕ may be particularly useful inmultipurpose laser systems capable of performing different operations,such as ophthalmic laser systems for both anterior segment (e.g.,cataract) and corneal (e.g., refractive) surgeries. In a cataractprocedure (e.g., capsulorhexis or fragmentation), laser incisions aremade in and around the crystalline lens at depths between 4-12 mmrelative to an anterior surface eye. In contrast, a refractive procedure(e.g., e.g., flap cutting, lenticule removal, etc.) entails making laserincisions in the cornea at relatively shallow depths between 0.01-1 mmrelative to an anterior surface eye. Accordingly, in preparation forsuch a cataract procedure, the angle of polarization of beam 101 may beadjusted to ϕ=45° to split the energy of the pulsed beam equally betweenordinary and extraordinary beams spatially separated in depth along anoptical axis (z-axis) of the laser system. This effectively doubles therepetition rate of the laser at depth, reducing cataract procedure timeswithout increasing the actual repetition rate of the laser engine. Priorto a refractive procedure, the angle of polarization of beam 101 may beadjusted to ϕ=0° so that little or no energy of the pulsed beam is splitto either the ordinary and extraordinary beam (depending on theparticular implementation). Rotation to different positions may beperformed manually by a system operator or by signals sent by lasercontroller 160 in response to user commands or as part of an automaticconfiguration sequence for system 100. Accordingly, embodiments ofsystem 100 may automatically set the angle of polarization of beam 101(by rotating birefringent lens 220 or a halfwave plate) to a cataractposition (ϕ=45°) during calibration prior to a cataract procedure, andalternatively to a refractive position (ϕ=0°) curing calibration priorto a refractive procedure.

It is further noted that adjusting the relative energy content of theordinary and extraordinary beams may be useful to influence theinteraction of multiple, near-photodisruption-induced cavitation bubblescaused by spots 250 o and 250 e. In general, when two cavitation bubblesare sufficiently close in space and time to interact, a strong jet ofbubbles is formed in the vertical direction (along optical axis 250).The direction of the jet propagation (i.e., the direction along opticalaxis 250) can be precisely controlled by the relative energy between theordinary and extraordinary focus spots 250 o and 250 e. The physics ofcavitation bubble interaction is such that the jet is formed toward thesmaller bubble. Thus distributing less energy for the extraordinaryfocus spot 250 e depicted in FIG. 2, the jet will be directed away fromthe source of beam 101. In this way, unwanted interaction of the jetwith incident laser pulses 250 o, 250 e can be eliminated. Thepolarization angle of beam 101 may thus be rotated in the mannerdescribed to control the interaction of cavitation bubbles during aprocedure, such as a lens fragmentation or capsulorhexis.

Returning to FIG. 2, in an example embodiment lens 210 comprises BK7 andhas a focal length f₀=1425.6 mm. Birefringent lens 220 is made ofcalcite with an ordinary refractive index n_(o)=1.658, an extraordinaryrefractive index n_(e)=1.486, and a central thickness of 1.744 mm. Lens210 and birefringent lens 220 are designed such that compound lens 200has zero optical power for extraordinary beam 101 e and +0.25 diopterpower for the ordinary beam 101 o. In such a configuration, passing beam101 through compound lens 200 in which lens 210 is BK7 and birefringentlens 220 is calcite splits the beam into an ordinary beam 101 o having acurved wavefront with a 4 meter radius of curvature, and anextraordinary beam 101 e having a flat wavefront. Moreover, the curvedordinary wavefront 201 o is temporally delayed relative to the flatextraordinary wavefront 201 e. In calcite, n_(o)<n_(e), and the ordinarybeam 101 o will be delayed with respect to the extraordinary beam 101 eby approximately 1000 femtoseconds. Beams 101 o and 101 e propagate to a5× beam expander 230 which increases the radius of curvature of ordinarybeam wavefront 201 o from 4 meters to 100 meters, while the radius ofcurvature of extraordinary beam wavefront 201 e remains flat. Beams 101o and 101 e are then focused by objective 250 having a focal length F=50mm. Accordingly, ordinary beam focus spot 250 o and extraordinary beamfocus spot 250 e will be separated by distance Δ:

Δ=[(n _(o) −n _(e))/(n _(e)−1)]*F ²/(M ² /f ₀):

Δ=(0.172/0.486)*50²/(5²/1415.6)=25 μm

In this example, delivery of ordinary beam 101 o is delayed with respectto the extraordinary beam wavefront 101 e by approximately 1000femtoseconds. Thus, in this example, an extraordinary pulse is deliveredbefore the ordinary pulse to a given x-y position at a particularz-depth. That is, ordinary pulse is focused at approximately the samex-y position as the extraordinary pulse, but at a z-depth which is 25 μmcloser to the anterior surface of the target, and approximately 1000femtoseconds after delivery of the extraordinary pulse. As a result,plasma will develop at the ordinary beam focus spot 250 o only after theextraordinary beam focus spot 250 e has been delivered, avoiding adverseeffects of shadowing. Temporal delay between the ordinary andextraordinary breakdowns can also be achieved by inserting a properlyoriented birefringent slab of proper thickness into the beam before thecompound lens 200.

Such a configuration (and similar variants) may be well-suited forophthalmic applications, as it is known that photodisruption-inducedcavitation bubbles generated in succession (e.g., within 5000femtoseconds) at close distances (e.g., within 20-30 μm) interact witheach other, increasing cutting efficiency in optical tissue. Inparticular, placing two focal spots approximately 25 μm apart inophthalmic tissue in less than 5000 femtoseconds may produce a cuttingeffect which exceeds the effect of two spots of comparable energyapplied further apart in time or distance. Accordingly, componentsdepicted in FIG. 2 may be optimized to generate ordinary beam focus spot250 o and extraordinary beam focus spot 250 e which are spatiallyseparated in depth along optical axis 250 by a distance of approximately25 μm, or in the range of 20-30 μm, and temporally separated by a delaygreater than or equal to a pulse duration of the femtosecond laserpulses (e.g., 100-10,000 femtoseconds).

It is noted that parameters described in the examples above are providedto illustrate a particular embodiment and do not limit scope ofdisclosure. One skilled in the art will appreciate that various otherconfigurations (material combinations, lens powers, etc.) will besuitable to optimize performance for a particular application. Althoughapplication ophthalmic procedures such lens fragmentation andcapsulorhexis are discussed in detail, it is contemplated thatembodiments may be useful for other ophthalmic procedures, includingcorneal procedures, vitreo-retinal procedures, or other procedures.Embodiments may also be useful outside the ophthalmic context, toconfigure a femtosecond laser to generate two focal spots spatiallyseparated in depth along an optical axis and temporally separated by adelay.

FIG. 4 depicts a method 400 for performing a surgical procedure usinglaser system 100, according to certain embodiments. At step 402, arelative energy distribution between an ordinary and extraordinary beammay be set to a cataract setting. As described above with respect toFIG. 2, the angle of polarization of beam 101 may be adjusted to ϕ=45°(by rotating birefringent lens 220 or a halfwave plate) to split theenergy of a pulsed beam equally between ordinary and extraordinary beamsspatially separated in depth along an optical axis (z-axis) of the lasersystem. This effectively doubles the repetition rate of the laser atdepth, reducing cataract procedure times without increasing the actualrepetition rate of the laser engine. By using the Z doubling techniquethe laser scanners can also run at half the speed otherwise required toachieve an analogous surgical effect. Alternatively, the relative energydistribution may be set to a corneal setting such that only one of theordinary and extraordinary foci has sufficient energy to causephotodisruption. This is useful for procedures in which incision depthsare relatively small. The energy distribution setting may be based onprocedure identification and specifications input by a system operatorvia a user interface.

In certain embodiments, a system operator may initiate a systemcalibration or configuration process for system 100 to automatically setthe angle of polarization of beam 101 to a desired setting. For example,laser controller 160 may receive signals indicating a cataract procedure(e.g., capsulotomy, lens fragmentation, etc.) or corneal procedure(flap, lenticule, tunnel, ring, etc.) is to be performed and in turnsend signals to an actuator configured to rotate birefringent lens 220or a halfwave plate to the requisite position.

At step 404, laser controller 160 generates a scan pattern consistentwith the setting selected at step 402. In certain examples, lasercontroller 160 may analyze biometric data, an OCT image of a target, orother information and, based on the information and selected setting,generate a scan pattern for a surgical procedure that accounts for thephotodisruptive effect of the ordinary and extraordinary focus spots.The scan pattern may be, for example a scan pattern for a lensfragmentation procedure, a capsulotomy, a LASIK incision, a lenticularincision, etc. In some examples, the scan pattern may be a spiralpattern, spider pattern, elliptical pattern, or any other suitablepattern.

At step 406, a beam of femtosecond laser pulses is generated by laserengine 110. Each laser pulse may have a total energy that is at least 2×the photodisruption threshold of the target ophthalmic tissue, so thateach ordinary and extraordinary pulse produced at step 410 hassufficient energy to induce photodisruption independently.

At step 408 the generated pulses are scanned in three dimensions by alaser scanner 120 according to the generated scan pattern. At step 410,each pulse of the generated laser beam is split into an ordinary andextraordinary pulse with approximately equal energy (e.g., a 50/50energy split, ±3%). A compound (e.g., doublet, triplet, etc.) lens 200comprising a lens 120 and a birefringent lens 220 may be used asdescribed with respect to FIGS. 2, 3A and 3B. As a result, compound lens200 generates an ordinary beam comprising ordinary pulses and anextraordinary beam comprising extraordinary pulses. A particularordinary pulse and a particular extraordinary pulse resulting fromsplitting a particular laser pulse may be spatially separated in depth,along an optical axis of the compound lens 200, by a distance greaterthan or equal to 5 μm (e.g., 15-35 μm), and may be temporally separatedby a delay greater than or equal to a pulse duration of the laser pulse(e.g., 100-10,000 femtoseconds).

At step 412, the ordinary and extraordinary beams are delivered into thetarget to perform the cataract procedure. Delivery may entail passingthe beams through a beam expander 230 and objective 240 as shown in FIG.2. As a result of the pulse energy, spatial separation, and temporalseparation, an extraordinary pulse (of the extraordinary beam) causesphotodisruption at a first depth in the target tissue before theordinary pulse (of the ordinary beam) causes photodisruption at asecond, greater depth in the target tissue (depth being relative to anexternal surface of the ophthalmic target). The cataract procedure maybe, for example, a lens fragmentation and/or capsulorhexis procedure.

At step 414, a relative energy distribution between an ordinary andextraordinary beam may be set to a refractive setting. As describedabove with respect to FIG. 2, the angle of polarization of beam 101 maybe adjusted to ϕ=0° (by rotating birefringent lens 220 or a halfwaveplate) to minimize or eliminate the split of energy between ordinary andextraordinary beams. In certain embodiments, the ϕ=0° polarizationposition distribute at least 95%, 98%, or 99% of the relative energy toeither the ordinary or extraordinary beam.

In certain embodiments, a system operator may initiate a systemcalibration or configuration process for system 100 to automatically setthe angle of polarization of beam 101 to a refractive setting or anyother suitable procedure setting. For example, laser controller 160 mayreceive signals indicating a refractive procedure is to be performed andin turn send signals to an actuator configured to rotate birefringentlens 220 or a halfwave plate to the requisite position for a cornealprocedure.

At step 416, laser controller 160 generates a scan pattern based on therefractive setting selected above. In this regard, laser controller 160may generate a refractive scan pattern that will not utilizephotodisruptive effects of dual focus spots. The scan pattern may, forexample, be configured to define a corneal flap, lenticule, tunnel, etc.At steps 418-422, a beam of femtosecond laser pulses is generated,scanned, and delivered to the cornea to perform a laser-assistedrefractive procedure.

One skilled in the art will appreciate that various steps of method 400may be optional, and may be performed in different order. For example,steps 402-412 may be performed without steps 414-422. Likewise, steps414-422 may be performed without steps 402-412.

Accordingly, embodiments of the disclosure utilize a compound lens witha birefringent lens component, together with other optical elements, tosplit the energy of a femtosecond laser beam pulse into an ordinary andextraordinary beam, each with an energy sufficient to inducephotodisruption in ophthalmic tissue. The two beams may be focused beloweach other in a target with a spatial separation in the range of a fewtens of a micron and a temporal delay commensurate with the pulse lengthof the beams. Thus, each laser spot can form a breakdown cavity intissue under treatment. In this way, the effective laser repetition rateof a laser system can be doubled without increasing the repetition rateof the laser engine.

The techniques and methods of the disclosure thus provide numerousadvantages and benefits. For example, a birefringent double lens such ascompound lens 200 can be implemented with an existing laser engine andscanners at relatively low cost. Further, embodiments may effectivelydouble the efficiency of a laser source without any increase of theactual repetition rate of the laser engine, thereby avoiding thetechnical challenges associated with a 100% increase in repetition rate(both in the laser source and scanners). As a result, embodiments of thedisclosure provide an inexpensive, reliable optical improvement that canpotentially reduce procedure times of ophthalmic procedures by 50% ormore, using existing laser equipment at comparatively low cost.

Moreover, because embodiments are configured to split a beam intoordinary and extraordinary pulses separated in depth, they are suitablefor use in processes which use scan patterns (e.g., spiral, circular, orelliptic cylinder patterns) that entail cutting at various z-depths(along the optical axis) along lines in the x-y dimension (perpendicularto the optical axis). In contrast, prior multiplexing solutions split abeam into two spots to different points in an x-y plane, but at the samez-depth, and are were thus unsuitable for spiral scan patterns employedby commercial ophthalmic laser systems, including LenSx®.

Additionally, embodiments of the disclosure allow the beam-splittingfunctionality to be selectively enabled or disabled by rotating anoptical component (e.g. the birefringent lens or a halfwave plate)manually or automatically by an actuator under the control of a lasercontroller. Accordingly, various embodiments can be easily and quicklyconfigured to different modes, such as a cataract mode and a refractivemode, which enable or disable splitting of beams into ordinary andextraordinary beams. These and other benefits will be apparent to oneskilled in the art in view of the specification, figures, and claims.

While several of the examples discussed above are described in terms ofophthalmic applications, such as cataract surgery, capsulotomy orcorneal procedures, implementations of system 100 can be used in aremarkably wide range of applications, which include a wide variety ofophthalmic procedures, such as retinal and corneal surgery, as well asdermatological and dental applications, different surgical applications,and various material machining applications, which shape a piece ofmaterial with laser photodisruption or some other laser aided process.

Accordingly, the foregoing descriptions of various embodiments have beenpresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the variantsdisclosed. Modifications and variations will be apparent topractitioners skilled in the art, and the present disclosure is intendedto encompass such modifications and variations.

1. An ophthalmic laser system, comprising: a laser engine configured togenerate a beam of femtosecond laser pulses; a laser scanner configuredto scan each laser pulse of the beam in three dimensions according to ascan pattern; a compound lens comprising a glass lens and at least onebirefringent lens, the compound lens arranged to receive the scannedbeam and configured to split each laser pulse of the scanned beam intoan ordinary pulse and an extraordinary pulse, thereby producing anordinary beam comprising ordinary pulses and an extraordinary beamcomprising extraordinary pulses, wherein a particular ordinary pulse anda particular extraordinary pulse split from a particular laser pulse arespatially separated in depth along an optical axis of the compound lens,by a distance greater than or equal to 5 μm, and temporally separated bya delay greater than or equal to a pulse duration of the femtosecondlaser pulses; and an objective configured to focus the ordinary beam andthe extraordinary beam within an ophthalmic target.
 2. The system ofclaim 1, wherein the scan pattern comprises a spiral scan pattern. 3.The system of claim 1, wherein the birefringent lens comprises abirefringent crystal.
 4. The system of claim 3, further comprising: arotation lens mount coupled to the birefringent lens; and wherein thebirefringent lens is rotatable to adjust a relative energy content ofthe ordinary beam and the extraordinary beam.
 5. The system of claim 4,wherein the birefringent lens is configured such that: rotating thebirefringent lens to a first position causes the relative energy contentbetween the ordinary beam and the extraordinary beam to be equallydistributed; and rotating the birefringent lens to a second positioncauses at least 95% of the relative energy to be distributed to theordinary beam and no more than 5% of the relative energy to bedistributed to the extraordinary beam.
 6. The system of claim 3, furthercomprising: a rotation mount coupled to a halfwave plate positionedbefore the scanner and the birefringent lens; and wherein the halfwaveplate is rotatable to change an orientation of polarization of thescanned beam and thereby adjust the relative energy content of theordinary beam and the extraordinary beam.
 7. The system of claim 6,wherein the halfwave plate is configured such that: rotating thehalfwave plate to a first position causes the relative energy contentbetween the ordinary beam and the extraordinary beam to be equallydistributed; and rotating the halfwave plate to a second position causesat least 95% of the relative energy to be distributed to the ordinarybeam and no more than 5% of the relative energy to be distributed to theextraordinary beam.
 8. The system of claim 3, wherein: the spatialseparation in depth of the ordinary and extraordinary pulses along theoptical axis is in the range of 5-100 μm; and the delay temporallyseparating the ordinary and extraordinary pulses is in the range of100-10,000 femtoseconds.
 9. The system of claim 4, further comprising alaser controller configured to: generate a scan pattern based on arotation position of the birefringent lens; and control the scanner toscan the beam according to the generated scan pattern.
 10. The system ofclaim 3, wherein the laser engine is configured to generate the beam offemtosecond laser pulses with a pulse energy sufficient to causephotodisruption in an ophthalmic target at focus spots of the ordinarybeam and the extraordinary beam.
 11. The system of claim 10, wherein thecompound lens is configured so that the spatial and temporal separationbetween the particular ordinary pulse and particular extraordinary pulseis sufficient to cause a photodisruption at a first depth in theophthalmic target before causing photodisruption at a second depth inthe ophthalmic target, wherein the first depth is greater than thesecond depth relative to an external surface of the ophthalmic target.12. The system of claim 3, wherein the birefringent lens comprises atleast one of calcite (CaCO₃), ytrium orthovanadate (YVO₄), lithiumniobate (LiNbO₃), or barium borate (BaB₂O₄).
 13. A method of performingophthalmic surgery, comprising: generating a beam of femtosecond laserpulses; scanning each laser pulse of the generated beam in threedimensions according to a scan pattern; splitting, with a compound lenscomprising a glass lens and at least one birefringent lens, each laserpulse of the scanned beam into an ordinary pulse and an extraordinarypulse, thereby generating an ordinary beam comprising ordinary pulsesand an extraordinary beam comprising extraordinary pulses; wherein aparticular ordinary pulse and a particular extraordinary pulse resultingfrom splitting a particular laser pulse are spatially separated indepth, along an optical axis of the compound lens, by a distance greaterthan or equal to 5 μm, and temporally separated by a delay greater thanor equal to a pulse duration of the laser pulse; and delivering theordinary beam and the extraordinary beam into an ophthalmic targetaccording to the scan pattern such that the particular extraordinarypulse causes photodisruption at a first depth in the target tissuebefore the particular ordinary pulse causes photodisruption at a seconddepth in the target tissue, wherein the first depth is greater than thesecond depth relative to an external surface of the ophthalmic target.14. The method of claim 13, wherein the scan pattern comprises a spiralscan pattern.
 15. The method of claim 13, wherein the birefringent lenscomprises a birefringent crystal.
 16. The method of claim 15, furthercomprising: rotating the birefringent lens or a polarization of thegenerated beam to adjust a relative energy content of the ordinary beamand the extraordinary beam.
 17. The method of claim 16, furthercomprising: rotating the birefringent lens or the polarization of thegenerated beam to a ϕ=45° polarization to equally distribute therelative energy content between the ordinary beam and the extraordinarybeam prior to a cataract procedure.
 18. The method of claim 16, furthercomprising: rotating the birefringent lens or the polarization of thegenerated beam to a ϕ=0° polarization to distribute at least 95% of therelative energy to the ordinary beam and no more than 5% of the relativeenergy to the extraordinary beam prior to a corneal procedure.
 19. Themethod of claim 15, wherein: the spatial separation in depth of theordinary and extraordinary pulses along the optical axis is in the rangeof 5 μm-100 μm; and the delay temporally separating the particularordinary and extraordinary pulses is in the range of 100-10,000femtoseconds.
 20. The method of claim 16, further comprising: generatingthe scan pattern based on a rotation position of the birefringent lens;and controlling a scanner to scan the beam according to the generatedscan pattern, wherein the scan pattern comprises a spiral scan pattern.